1. Field of the Invention
This invention relates to the architecture of a switching node that handles the D Channel out-of-band signaling protocol of an Integrated Services Digital Network (ISDN).
Typically, the application of such a node can be found in systems requiring D Channel message processing. More specifically, the present invention finds its primary utility in the area of digital networks implementing ISDN.
2. Glossary
The following terms may be used freely in this document:
______________________________________ ASP Assignment Source point. That entity in the network at layer 2 which manages the TEI values. B Channel The bearer channel, logically different from the D Channel. The B Channel does not carry any signaling in-band for call processing on ISDN. Basic Rate A defined user-network interface. Contains 2 B Interface Channels and a D Channel, logically multiplexed together at Layer 1 CCITT International Telegraph and Telephone Consultive Committee. An international body which establishes communication standards. CES Connection Endpoint Suffix. The Layer 3 identifier, similar to the TEI at Layer 2. The CES and TEI have a one-to-one correspondence. CO Central Office. DCE Data Circuit Terminating Equipment. D Channel The signaling channel in ISDN. 16 Kbps in the basic rate interface and 64Kbps in the primary rate interface. Demux Demultiplexer. DIP Dual Inline Package. DLCI The LAPD address. DLCI = SAPI + TEI. DSL Digital Subscriber Loop. The connection between the network node and the customer. DTE Data Terminal Equipment. IC Integrated Circuit. ISDN Integrated Service Digital Network. ISO International Standards Organization. An international body of technical experts which proposes and establishes technical standards. LAPB Link Access Protocol Balanced. LAPD Link Access Protocol on the D Channel of ISDN. LSB Least Significant Bit. MSB Most Significant Bit. Mux Multiplexer. Out-of- When a separate logical channel is used to transport Band the signaling information. Signaling OSI Open System Interconnection. Primary A defined user network interface containing 23 B Rate Channels (or 30 B Channels) and a D Channel Interface (64Kbps) logically multiplexed together at Layer 1. 2B+D Two Bearer (2B) channels for carrying User data plus one signaling (D) Channel used in basic rate ISDN. 23B+D 23 Bearer (23B) Channels for carrying User data plus one signaling (D) Channel used in Primary Rate ISDN. S,T,U Reference points definded in the ISDN user- References network interface. Peer Communicating Entities at the same given layer. SAPI Service Access Point Identifier. A part of the LAPD address. Identifies the type of Layer 3 entity associated with the cell. TEI Terminal Endpoint Identifier. A part of the LAPD address, identifying at Layer 2 the unique terminal that is associated with the cell information. ______________________________________
3. Background and Overview of ISDN
At this writing, ISDN is in the process of evolving into a telephony Integrated Digital Network that provides end-to-end digital connectivity to support a wide range of services, including voice and non-voice services, to which users have access by means of a limited set of standard multi-purpose user-network interfaces. An ISDN contains the intelligence for providing service features, maintenance and network management functions. The architecture of ISDN standards closely follows the OSI seven layer Reference Model, although these standards do not map exactly onto existing OSI protocols. Only the lower three layers of the OSI model are of primary interest in considering the present invention. The following background information is provided as a general overview of ISDN as it is currently proposed by various standards organizations and is not intended to be limiting to the present invention since the standards themselves are currently in the process of evolving and the present invention may be equally applicable to a number of variations of ISDN.
Two main entities exist in an ISDN: the network and the user. The network provides services (which are the communication capabilities made available to the customer by telecommunication service provider) and the user accesses these services through the user-network interface. A "channel" represents a specified portion of the information carrying capacity of an interface. Channels are classified by channel types. The channel types and their uses in Basic Rate ISDN are:
The B channel is a 64 Kbps channel accompanied by timing. It is intended to carry a wide variety of user information streams, and does not carry any signaling information for switching the B Channel.
The D Channel is a 16 Kbps channel for the Basic rate interface. It is primarily intended to carry signaling information for circuit switching by the ISDN. All the ISDN work that is being done with reference to signaling, refers only to the D Channel. The D Channel uses a layered protocol, and the data flow is only in packets.
The architecture of the present invention may be used at either basic rate or primary rate with suitable modifications evident to those skilled in the art. For simplicity of explanation, the bulk of the present description, both as to the background of ISDN and as to the invention itself, will be confined largely to basic rate. Those skilled in the art will appreciate that the present invention may be used at either basic rate, primary rate or at other rates dictated by the design of ISDN networks or future standards. The Basic Rate Interface consists of two B Channels and one D Channel, commonly referred to as "2B+D". All the signaling and some low speed data communication is done on the D Channel, while the bearer B Channels are used exclusively for data transport. This concept is called out-of-band signaling. Some of the advantages of out-of-band signaling are:
1. The entire bandwidth of the bearer channel is available for the user.
2. Uniform signaling procedures are available, without regard to the data type being transported on the bearer channel.
3. Since the signaling channel is separate, the response time is better than in-band signaling, and processes can be dedicated for signaling.
Those skilled in the art will recognize other advantages of out of band signaling.
Communication in an ISDN generally occurs between peer entities (for example, Physical Layer to Physical Layer communication, etc.), and between the layers (for example, between Layer 2 and Layer 3). Peer entities in the Data Link Layer communicate using the LAPD or Link Access Protocol on the D Channel. At the Network Layer (Layer 3), the CCITT Recommendations I.450/Q.930 and I.451/Q.931, which are incorporated by reference, define the applicable communication.
Peer entities at Layer 3 communicate using messages. Layer 3 is primarily responsible for the establishment, maintenance and disestablishment of each call, and each call is uniquely identified by a call reference number. Also, every call reference is associated with a Layer 3 entity called the Connection Endpoint Suffix (CES).
Each CES can support more than one call reference. Every Layer 3 message is packetized into a Layer 2 frame, the format of which is as shown in CCITT Recommendations I.440/Q.920 and I.441/Q.921 which are incoporated by reference. The address field (DLCI) is two bytes long and has two sub parts, viz., the Service Access Point Identifier (SAPI) and the Terminal Endpoint Identifier (TEI). The SAPI identifies the logical type of Layer 3 frame, (e.g., signaling, packet etc.), and the TEI identifies the particular terminal which is associated with this message. The TEI is assigned by the network node, either at initalization time, or when a call is being setup, and the entity making the assignment is the Assignment Source Point (ASP). The ASP is also responsible for removal of the TEI.
The D Channel and the B channel data streams are combined at Layer 1 into a format defined in CCITT Recommendation I.430 which is incorporated by reference. In case there is no useful data to transmit, a string of 1's is transmitted at Layer 1.
The functions of the lower three layers may be summarized as follows:
Layer 1: Activation, deactivation, port number record.
Layer 2: TEI management (by the ASP), timer and counter management, error detection and recovery management, link state management, buffer management, communication with Layers 1 and 3.
Layer 3: Call state management, communication with Layers 2 and 4, control of switch fabric for B channel switching, port number-directory number mapping.
When an ISDN terminal or device is powered up, Layer 1 goes through an activation sequence with the Layer 1 device at the network node, and synchronizes itself. When prompted by the user to establish a conneciton, the communication manager at Layer 2 communicates with the ASP, and gets a TEI assigned to itself. Using this TEI value, a Layer 2 link is established between the Layer 2 processes at the terminal and at the node. Once this is done, the Layer 3 processes communicate using the services of the lower two layers. It should be noted that at Layer 2, once a link is set up, flags are transmitted in between frames. When the signaling is complete, the bearer channels are `cut-through` by the network, and communciation as we know it can occur.
To tear down a connection, the Layer 3 peer entities disassociate themselves from each other, then the Layer 2 entities do the same. At this point, when no Layer 2 frames are given to Layer 1, a sequence of 1's is transmitted in the place of the D Channel bits.
The present invention provides a method and apparatus for implementing Layer 2 and Layer 3 processing in a network node providing an efficient modular system which can readily accommodate growth of the network. Some of the specific advantages offered by the architecture of the present invention are:
1. A modular architecture, providing easy change to the node, when the number of users or trunks change.
2. An architecture that supports minimum processing delay.
3. A plurality of processors are used to handle Layer 2 and Layer 3 processes with general purpose processors. An arbitration process is used to determine which processor is used for each process and the arbitration scheme used is dynamic (service based on the number of unsuccessful attempts to access the bus), based on the signaling load.
4. The architecture frees the implementor from using one type of processor. As long as the interface specification (gateway) is adhered to, virtually any appropriate processor, general purpose or dedicated, can be used.
5. The architecture allows for low cost implementation by using only the amount of processing power required by the data traffic supported by the node.
6. The node is easily upgraded by addition of processors as required to handle additional traffic.
7. The queues are arbitrated in a manner similar to that of the processors with priority determined by a priority number computed from queue length+number of unsuccessful bus accesses.